Method for preparing a molded glass material for use in the manufacture of living tissue replacement

ABSTRACT

A living tissue replacement of crystallized glass having bioaffinity and mechanical strength is briefly obtained simply by pressure molding or machining without using a special equipment. A glass material having a softening point below its crystallization temperature and exhibiting viscous flow at temperatures below its melting point is heated at a temperature above its Tg and pressed at the temperature to mold to a desired shape, thereby manufacturing a living tissue replacement such as a dental crown. Molding can be done under a pressure of up to 20 MPa.

This application is a Continuation of application Ser. No. 08/245,038,filed on May 17, 1994, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to living tissue replacements such as artificialdental crowns, artificial dental roots, artificial bones, bone screws,and artificial air tubes. It also relates to a method for preparingliving tissue replacements, a glass material and a molding apparatus foruse in the manufacture of living tissue replacements.

2. Prior Art

Various biotic ceramics are used to form living tissue replacements suchas artificial dental crowns, artificial dental roots, artificial bones,artificial junctions and bone fillers. Among biotic ceramics, greatattention is paid to crystallized glass or glass-ceramics because ofgood biological affinity and high mechanical strength. As to thecrystallized glass for biological use, the following proposals have beenmade.

Japanese Patent Publication (JP-B) No. 69094/1992 proposes a calciumphosphate system crystallized glass for use as a dental materialcomprising CaO, P₂ O₅, and Al, with the Ca/P ratio being from 0.35 to0.49. A shape of glass is formed by centrifugal casting and then heattreated for crystallization. Since this method utilizes centrifugalcasting for shaping of glass, the glass must be melted, making itdifficult to use a high strength glass composition having a high meltingpoint. Then, the crystallized glass of this proposal is less reliablewhen applied to living tissue replacements which receive substantialimpacts in a repetitive manner. In fact, the calcium phosphate systemcrystallized glass examples disclosed in the publication are notregarded satisfactory in mechanical strength and a substantial amount ofglass component leaches out. The casting process is difficult to ensuredimensional precision since glass experiences considerable shrinkageupon cooling. The casting process also allows bubbles to be introducedinto the glass which is then insufficient in strength, often resultingin defective parts due to bubble inclusion. The crystallized glass ofthis proposal raises problems particularly when applied to artificialdental crowns. While artificial dental roots need not have a shape andsize specific to an individual patient so that standard parts can bemanufactured on a large scale, artificial dental crowns must beconfigured to a shape conformal to the deficient site of an individualpatient and are thus required to be easily shaped by the dentist ordental technician with simple means. It is, however, difficult for thedentist or dental technician to perform shaping by a casting process.The casting process further has the problem that heat treatment over along time is necessary since crystallization of glass takes place froman amorphous state after shaping by the casting process. In fact,crystallization took about 10 to 20 hours in the examples disclosed inthe publication.

JP-B 36107/1992 discloses crystallized glass for use as artificial bonesand dental materials. This crystallized glass has a non-calciumphosphate system composition free of P₂ O₅. It is prepared by moldingglass powder, followed by firing and crystallization treatment. With theprocess of molding and firing glass powder, it is difficult to preparedental crowns and other parts of complex shape. Since the molding stepuses an isostatic press and the firing step uses a high temperature of1,050° C. as described in the publication, this process is quitedifficult to practice in the dental office. The process takes a longtime since the firing step uses a slow heating rate of 30 to 60°C./hourand a slow cooling rate of 30 to 120° C./hour. Dimensional precision islow since firing entails a large shrinkage factor. Even when a highstrength composition is used, firing of glass powder after molding tendsto lower strength. Preparation of glass powder requires cumbersomeoperation and considerable costs because molten glass must be convertedinto ribbon shape as by passing through water-cooled rollers. Inaddition, many voids are left after firing.

Furthermore, wollastonite and diopside are precipitated in thecrystallized glass of JP-B 36107/1992 although the amount of diopsideprecipitated is not specified therein except for only one datum of 40%in Example.

Japanese Patent Application Kokai (JP-A) No. 70244/1987 discloses adental crown-forming material comprising crystallized glass. It isprepared by casting a molten raw material into a mold and heat treatingthe molded material. At the end of heat treatment, Na.Mg₃ (Si₃ AlO₁₀)F₂grains (mica) having improved mechanical workability and Li₂ O.Al₂O₃.2SiO₂ grains (β-eucryptite) and Li₂ O.Al₂ O₃.4SiO₂ grains(β-spodumene) having improved mechanical strength precipitate in thisdental crown-forming material. TiO₂ and ZrO₂ are added for controllingcrystal growth and improving mechanical strength and Fe₂ O₃ and MnO areadded to control color. The examples disclosed in this publicationachieve a flexural strength of 2,000 to 2,700 kg/cm², which is stillinsufficient. The use of a casting process for molding suffers fromproblems as mentioned above.

JP-A 12637/1987 discloses a glass ceramic dental crown which is preparedby molding molten glass followed by heat treatment for causing mica andspodumene crystal phases to precipitate out. Although the glass ceramicis alleged to be improved in machinability and mechanical strength, noexemplary evaluation of machinability and mechanical strength isdisclosed. The use of a casting process for molding suffers fromproblems as mentioned above.

JP-A 174340/1991 discloses an artificial dental crown formed of a glassceramic composition comprising a calcium-potassium mica crystal and atleast one of enstatite, akermanite, and diopside crystals, or comprisinga calcium-potassium-sodium mica crystal and at least one of enstatite,akermanite, diopside, anorthite, and richterite crystals. This glassceramic is alleged to have a hardness approximate to natural teeth andimproved mechanical strength, mechanical workability, corrosionresistance and light transmittance. The glass ceramic of such a crystalstructure has insufficient gloss and raises an aesthetic problem whenused as dental crowns. Also bioactivity is insufficient. The publicationdescribes that the step of shaping glass ceramic to a dental crownconfiguration includes a casting process as well as machining althoughthe casting process raises problems as mentioned above.

JP-A 88744/1991 discloses a glass ceramic composition comprising abarium-calcium mica crystal and at least one of enstatite, forsterite,and diopside crystals or comprising a barium-calcium mica crystal and atleast one of enstatite, forsterite, diopside, and tetragonal zirconiacrystals. In the examples therein, a maximum flexural strength of 5,000kg/cm² is reported. However, this glass ceramic is intended formachinable ceramic, but not for application to biotic materials such asartificial dental crowns. In fact, the glass ceramic of such a crystalstructure has insufficient gloss and raises an aesthetic problem whenused as dental crowns.

SUMMARY OF THE INVENTION

An object of the present invention is to make it possible to prepare aliving tissue replacement of crystallized glass having both biologicalaffinity and mechanical strength without a special mechanical means andwithin an acceptably short time. Another object of the invention is tomake it possible to prepare a living tissue replacement of crystallizedglass having both biological affinity and mechanical strength bymechanical working in a simple manner.

According to a first aspect, the present invention provides a glassmaterial for use in the manufacture of a living tissue replacement,which has a crystallization temperature and a softening point which islower than the crystallization temperature and exhibits viscous flow attemperatures below its melting point.

Preferred embodiments are described below.

Preferably, the glass material has a non-calcium phosphate systemcomposition comprising silicon oxide, calcium oxide, and magnesiumoxide. The total content of silicon oxide, calcium oxide, and magnesiumoxide, calculated as SiO₂, CaO, and MgO, respectively, is at least 70%by weight, more preferably at least 80% by weight of the composition.The contents of the respective components are 40 to 70% by weight ofSiO₂, 20 to 50% by weight of CaO, and 8 to 30% by weight of MgO based onthe total content.

Preferably, the glass material further contains at least one elementselected from the group consisting of Na, K, B, Al, Ba, Fe, Zr, Ce, Au,Ag, Cu, Ti, Cr, Ni, Li, Bi, Co, V, Pd, Pt, Sn, Sb, F, Mn, Sr, Nb, Ta, Y,and Ca. More preferably, the glass material contains up to 20% by weightof TiO₂ or up to 10% by weight of ZrO₂.

Preferably, the glass material has a crystallization temperature of upto 1,000° C. The glass material exhibits a distortion of at least 20% ata temperature of up to 1,000° C. and a pressure of up to 20 MPa.

Typically, the living tissue replacement is an artificial dental crown.

According to another aspect, the invention provides a method forpreparing a living tissue replacement comprising the step of molding aglass material at a temperature below its melting point under pressureby utilizing a viscous flow phenomenon.

Preferred embodiments are described below.

Preferably, the glass material prior to pressure molding has a firstcrystallinity and the living tissue replacement has a secondcrystallinity greater than the first crystallinity. The glass materialis typically as set forth in the first aspect.

Preferably, the method further includes the step of effectingcrystallization treatment on the glass material during or after thepressure molding. The method may further include the step of nucleatingthe glass material prior to the crystallization treatment. Morepreferably, the glass material is nucleated, thereafter pressure moldedat a temperature not lower than its glass transition temperature, andcrystallized during or after the pressure molding. Typically, pressuremolding uses a pressure of up to 20 MPa and a temperature which is nothigher than the crystallization temperature plus 50° C. Preferably, theglass material during pressure molding has a crystallinity of up to 50%by volume. Preferably, the glass material is pressure molded at atemperature which is up to 0.8 times its melting point. Preferably, theglass material during pressure molding has a viscosity of up to 10⁹poise. At the end of pressure molding of the glass material, thepressure is released while the glass material is at a temperature notlower than its glass transition temperature.

In one preferred embodiment, the method uses a molding apparatusincluding a mold and a punch. The mold includes a molding cavity and abore through which the punch is inserted, the bore being in fluidcommunication with the molding cavity through a sprue. Then the glassmaterial is placed in the bore and the punch is urged into the boreagainst the glass material for effecting pressure molding. The moldfurther includes a vent in fluid communication with the molding cavity.The bore is defined by an inner surface which extends substantiallyparallel to the pressure applying direction. The bore inner surface hasa taper of up to 1/5. The sprue is inclined relative to the pressureapplying direction. The sprue has a cross-sectional shape correspondingto the shape of the molding cavity. The mold may further include a linercovering at least a portion of the bore inner surface, the liner beingmade of a high strength material having a higher compression strengththan the mold body. The punch may further include a cover forming atleast a portion of the surface of the punch opposed to the bore innersurface, the cover being made of a high strength material having ahigher compression strength than the mold body. The mold body has acompression strength of up to 20 MPa at the end of pressure molding. Thehigh strength material has a compression strength of at least 15 MPa.

In a further preferred embodiment, the method for preparing a livingtissue replacement further includes the step of machining the glassmaterial after the crystallization treatment.

According to a third aspect, the present invention provides a method forpreparing a living tissue replacement comprising the steps of:crystallizing a glass material as defined in the first aspect andmachining the crystallized glass material.

Also contemplated herein are living tissue replacements prepared by theabove mentioned methods. Typically, diopside grains are dispersed in theliving tissue replacement.

Also contemplated herein is a living tissue replacement moldingapparatus which is used in the molding step of the above-mentionedmethod for preparing a living tissue replacement.

ADVANTAGES

According to the invention, a living tissue replacement is prepared bymolding a glass material at a temperature between its glass transitiontemperature (inclusive) and its melting point (exclusive) under acertain pressure into a desired shape such as a dental crown whileutilizing the viscous flow phenomenon that the glass exhibits at thetemperature. By utilizing the viscous flow, the glass material can bemolded to a desired shape under a pressure of up to about 20 MPa.

The glass material used herein is preferably of a composition capable ofcreating diopside (CaO.MgO.2SiO₂) crystals. We have found that glass ofthis composition is a bioactive material which has a low viscosity attemperatures from its glass transition temperature to near itscrystallization temperature, especially from its softening point to nearits crystallization temperature so that it is prone to pressure molding.Glass materials having a composition within the scope of JP-B 36107/1992are not sufficiently low in viscosity at such temperatures.

The present invention has the following advantages.

(1) Glass material can be molded under pressure at a temperature oflower than its melting point, preferably at a temperature lower thannearly its crystallization temperature, for example, 1,000° C. or lower,especially 900° C. or lower. Then the glass material can be heated inconventional furnaces which are commonly found in dental offices. Theglass material can be molded in air without any deterioration byoxidation, eliminating a need for atmosphere control. The glass materialprovides smooth mold release. Even when metal compounds are used as acoloring agent, they are not burnt.

(2) Glass material can be molded under a low pressure of up to 20 MPa,especially up to 1 MPa. This eliminates a need for special pressurizingmeans and enables molding by merely using a hand press or weight, forexample. A high strength mold is no longer needed. The glass materialcan be easily molded in ordinary dental offices.

(3) The present invention prefers the use of crystallized glass having acomposition in the diopside field, which has extremely high strength,but an extremely high melting point (often above 1,4000° C.) so that itscasting is almost impossible. The pressure molding process of theinvention can form crystallized glass having a composition in thediopside field without heating to extremely high temperatures. Thecrystallized glass having a composition in the diopside field is sobioactive that it may be adequate as living tissue replacements.

(4) As opposed to a process of molding and firing glass powder, thepresent invention requires neither special mold nor special equipmentlike a continuous isostatic press (CIP). Conventional dental castingmolds can be used.

(5) The prior art process of molding and firing glass powder isdifficult to produce parts to precise dimensions because of the presenceof voids in the molded parts and a substantial shrinkage factor uponfiring. The present invention eliminates voids since glass material ismolded in bulk under pressure. The glass used herein has a very lowcoefficient of thermal expansion. There are thus available molded partsor living tissue replacements reflecting the mold cavity faithfully,that is, at high precision.

(6) The prior art process of molding glass powder is difficult toproduce complex shaped parts like dental crowns whereas the presentinvention is effective for producing complex shaped parts as easily asthe casting process because glass material is softened and fluidized inbulk. The molded parts are satisfactorily homogeneous. As opposed to thecasting process, few bubbles are found in the molded parts which havehigh strength and the percentage of defective parts is minimized.

(7) As opposed to the casting process, the glass material is not melted.It is then possible to effect nucleation and/or crystallization on theglass material prior to molding as shown in FIGS. 1(a), 1(d) and 1(e).More particularly, when the invention is applied to the manufacture of adental crown, the glass material which has been nucleated and/orcrystallized can be delivered to the dentist or dental technician,enabling substantial reduction of the time taken from molding tocompletion of a dental crown. The glass material of the invention has ashort crystallization time so that crystallized glass parts can beproduced within a short time even when crystallization is effectedduring or after pressure molding as shown in FIGS. 1(b) and 1(c). Forexample, a prior art process of molding and crystallizing amorphousglass required about 6 to 12 hours whereas the present invention cancomplete a dental crown of crystallized glass within only about 3 hours.

(8) The invention insures sufficiently high strength because homogeneousglass is directly molded and crystallized without melting. If part ofglass material is once melted upon molding, there is a likelihood thatthe glass as crystallized would not have a homogeneous grain structureand hence, sufficient strength. By molding glass material at atemperature which is lower than 0.8 times its melting point, localmelting of the glass material can be restrained nearly completely. Thepresent invention also eliminates a likelihood that glass be devitrifiedby melting or be distorted to lose strength upon cooling.

JP-A 231655/1987 discloses preparation of a dental instrument by moldinga ceramic material or alloy which can be plasticized by heating. Noreference is made to crystallized glass therein. In one example of thispublication, a dental crown is prepared by molding a mixture of aglass-forming base material, an aluminum oxide for imparting necessarystrength, a flux selected from K₂ O, Na₂ CO₃, CaO, and B₂ O₃, and aplasticizer such as glycerin. The example uses aluminum oxide forinsuring strength probably because the glass is not crystallized.Nevertheless, the thus obtained material has insufficient strength ascompared with the crystallized glass obtained in the present invention.Even if one attempts to crystallize the material of this publication, itis difficult to achieve high strength because inclusion of the fluxrestrains homogeneous crystallization. If a metal compound is added toglass as a coloring agent as is often the case, a heating temperature infar excess of 1,000° C. upon compression as described in the publicationcauses sublimation of the coloring agent, failing to provide a desiredcolor.

According to the present invention, a specific amount of TiO₂ is addedto a glass material comprised of SiO₂, CaO, and MgO as main componentsfor achieving a substantial improvement in machinability. Strength issignificantly improved by adding ZrO₂ along with TiO₂. Glass material ofsuch composition is easy in machining or mechanical working and ensuresprecise dimensions after machining. Therefore, the glass material lendsitself to precision machining utilizing a CAD/CAM system and is adequatefor the manufacture of artificial dental crowns and roots. When theglass material is processed by the pressure molding method utilizingviscous flow as defined herein, parts of any desired dimensions areobtained without sacrificing strength. Waste of the glass material uponmachining can be minimized, contributing to a cost reduction.

In the apparatus for molding glass material, a bore 23 through which apunch 4 is inserted has an inner surface which is slightly tapered ornot tapered as shown in FIGS. 3 to 5 for preventing back flow ofsoftened glass material 3 through a gap between the bore 23 and thepunch 4. This minimizes the necessary loading of glass material.

Also a mold 2 having a cavity 21 and a vent 24 in fluid communicationtherewith as shown in FIGS. 3 to 5 allows part of the glass material toescape under pressure so that the mold cavity may be readily filled withthe glass material, facilitating production of a molded shape faithfulto the mold cavity under a low pressure. Since the vent 24 preventsapplication of excessive pressure to the mold 2 during pressure molding,it is also effective for preventing the mold 2 from cracking, resultingin a drastic drop of percent generation of defective parts.

In an embodiment wherein a sprue 22 connecting the molding cavity 21 andthe punch-receiving bore 23 in fluid communication is inclined relativeto the pressing direction as shown in FIG. 5, the mold cavity 21 isreadily filled with the glass material, facilitating production of amolded shape faithful to the mold cavity under a low pressure. It isalso effective for preventing the mold 2 from cracking during pressuremolding.

The preferred embodiment where the sprue 22 has a cross-sectional shapecorresponding to the shape of the molding cavity facilitates productionof a molded shape more faithful to the mold cavity under a low pressure.

In the preferred embodiment wherein the mold 2 further includes a liner25 covering at least a portion of the bore 23 inner surface, the linerbeing made of a high strength material having a higher compressionstrength than the mold body as shown in FIGS. 10 to 12, the highstrength liner 25 is effective for preventing the mold 2 from crackingand the mold body made of a material having a relatively low compressionstrength is effective for preventing rupture of the molded part uponwithdrawal from the mold. In the further preferred embodiment wherein areinforcement 45 forms at least a portion of the surface of the punch 4opposed to the bore 23 inner surface, the reinforcement being made of ahigh strength material having a higher compression strength than themold body as shown in FIGS. 13(a), 13(b) and 13(c), any rupture of thepunch 4 is prevented during molding.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a) to 1(e) are flow diagrams showing steps of preparing a livingtissue replacement according to different embodiments of the invention.

FIGS. 2 to 5 are schematic sectional views of molds showing differentexemplary versions of pressure molding step.

FIG. 6 illustrates examples of the living tissue replacement accordingto the invention, FIG. 6(a) is an artificial vertebra body, FIG. 6(b) isan artificial intervertebral body, FIG. 6(c) is an artificial iliacbone, FIG. 6(d) is an artificial air tube, and FIG. 6(e) is a bonescrew.

FIG. 7 is a ternary diagram of SiO₂ --CaO--MgO system for explaining thecomposition of preferred ceramic material.

FIGS. 8 and 9 are graphs showing a distortion of glass material versusapplied pressure.

FIGS. 10, 11, and 12 are schematic sectional views of a mold including abore having a high strength liner on its inner surface.

FIGS. 13(a), 13(b) and 13(c) are cross-sectional views of a punchpartially formed of a high strength material.

FIG. 14 is a cross-sectional view illustrating how to provide the borewith a high strength liner.

FIG. 15 is a cross-sectional view illustrating how to prepare a punchpartially formed of a high strength material.

DETAILED DESCRIPTION OF THE INVENTION

Glass Material

The glass material which is used for the manufacture of living tissuereplacements according to the present invention is not particularlylimited in composition. A choice may be made of various compositionswhich are capable of pressure molding utilizing viscous flow. Exemplarypreferred compositions are those from which at least one ofCaO--MgO--SiO₂ (monticellite), 2CaO--MgO--2SiO₂ (akermanite),2(Mg,Ca)O--SiO₂ (forsterite), β-(Ca,Mg)O--SiO₂ (wollastonite), Na₂O--Al₂ O₃ --2SiO₂ (nephelite), Na₂ O--Al₂ O₃ --6SiO₂ (albite), Na₂O--Al₂ O₃ --4SiO₂ (jadeite), MgO--TiO₂, Al₂ O₃ --SiO₂ (andalusite), 3Al₂O₃ --2SiO₂ (mullite), Cao--Al₂ O₃ --2SiO₂ (anorthite), 2CaO--Al₂ O₃--SiO₂ (gehelenite), and 3CaO--Al₂ O₃ --3SiO₂ (grossularite) canprecipitate out. A glass material having a non-calcium phosphate systemcomposition comprising silicon oxide, calcium oxide, and magnesium oxideand having a softening point lower than its crystallization temperatureis preferred because of ease of pressure molding.

More preferably, in the glass material comprising silicon oxide, calciumoxide, and magnesium oxide, the total content of silicon oxide, calciumoxide, and magnesium oxide, calculated as SiO₂, CaO, and MgO,respectively, is at least 70%, especially at least 80% by weight of theentire glass material. The remainder is generally a coloring agent,crystallization promoter and the like. If the total content of SiO₂,CaO, and MgO is less than 70% by weight, some of the advantages of theinvention including bioactivity and viscous flow would be lost andstrength would become lower.

The contents of the respective components are

40 to 70%, especially 53 to 70% by weight of SiO₂,

20 to 50%, especially 20 to 35% by weight of CaO, and

8 to 30%, especially 10 to 25% by weight of MgO

based on the total content of SiO₂, CaO, and MgO. Outside the range, (1)a viscosity drop during pressure molding would be insufficient, leavinga likelihood of the glass material being broken under applied pressure;(2) vitrification would be retarded; (3) a necessary viscosity dropwould occur at higher temperature, to which the material must be heatedbefore pressure molding can be effected; (4) melting temperature wouldbecome higher; (5) the temperature at which diopside grains precipitatewould become higher; and (6) the type and amount of precipitating grainswould largely vary.

More particularly, a too low content of SiO₂ would retard vitrificationand lower strength whereas a too high content of SiO₂ would elevate themelting temperature. A too low content of CaO would retard grainprecipitation whereas a too high content of CaO tends to incurdevitrification and reduce the amount of diopside grains precipitated. Atoo low content of MgO would reduce the amount of diopside grainsprecipitated whereas a too high content of MgO tends to incurdevitrification.

In addition to these oxides, various elements or compounds may be addedto the glass material as a coloring agent, crystallization promoter orprocessing aid. Preferred is at least one element selected from thegroup consisting of Na, K, B, Al, Ba, Fe, Zr, Ce, Au, Ag, Cu, Ti, Cr,Ni, Li, Bi, Co, V, Pd, Pt, Sn, Sb, F, Mn, Sr, Nb, Ta, Y, and Ca. Amongthese elements, Al, Fe, Ce, Ag, Cu, Ti, Cr, F, Mn, Sr, Nb, Ta, Y and Cafunction as a coloring agent, Zr, Au, Ag, Pt, Ti and F function as acrystallization promoter, and Na, Li, Ti, K, B, and Al are effective forimproving processability. The processing aids are to facilitatevitrification and lower viscosity.

These additive elements may be added in elemental form or as compounds,with preferred exemplary compounds being oxides, chlorides, nitrates andsulfates.

More particularly, TiO₂ is advantageously added when the glass materialas crystallized is to be machined because addition of titanium dioxideis effective for improving strength and machinability and facilitatingcrystallization. It is also effective for improving aesthetic appearanceby imparting lustrous outer appearance as well as color tone and lighttransmittance similar to natural teeth. The glass material is thussuitable for dental crowns and artificial bones. Preferably the glassmaterial contains up to 20% by weight of TiO₂. A TiO₂ content of 7 to15% by weight is most preferred for increasing strength, with a flexuralstrength of higher than about 300 MPa being achieved for example. A toolow content of TiO₂ would be ineffective for its addition purpose, thatis, crystallization would be insufficient and strength would lower.

Preferably, ZrO₂ is added to the glass material along with TiO₂. Theaddition of ZrO₂ improves strength and machinability and is advantageouswhen the molded part is to be machined. The content of ZrO₂ ispreferably up to 10% by weight of the glass material, with maximumstrength being available at a ZrO₂ content of 0.1 to 2% by weight. A toolow content of ZrO₂ would be ineffective for its addition purpose, thatis, strength enhancement would be insufficient. A too high content ofZrO₂ would detract from machinability. The combined addition of TiO₂ andZrO₂ is effective for improving strength and machinability, with aflexural strength of higher than about 300 MPa being readily achievedfor example.

The glass material containing an adequate amount of TiO₂ or TiO₂ plusZrO₂ has high strength as mentioned above and is suitable for use inartificial bones and dental bridges as well as inlays and onlays.

By adding Sr, Nb, Ta, Y, Ca and Fe, the glass material can be colored toan appropriate tone as dental crowns. The content of these elements ispreferably up to 5% by weight in total provided that the elements arecalculated as SrO, Nb₂ O₃, Ta₂ O₅, Y₂ O₃, CaO, and Fe₂ O₃. In excess of5% by weight, strength would be lower.

Crystallization is promoted by adding silver and gold compounds such asAgCl, Ag₂ O and AuCl₃ in a total amount of 0.01 to 1.0% by weight.Excess addition of these compounds would cause undesirable coloring andover-crystallization.

The glass material has a softening point and a crystallizationtemperature wherein the softening point is lower than thecrystallization temperature, preferably by 20° C. or more, morepreferably by 50° C. or more. Pressure molding would sometimes bedifficult when the softening point is lower than the crystallizationtemperature by only a slight difference.

The glass material preferably has a crystallization temperature (Tx) ofup to 1,000° C., more preferably up to 900° C. and generally higher than600° C. The benefits of the invention are lost if the crystallizationtemperature is extremely high.

Possible molding and crystallization at temperatures below 1,000° C.eliminates the disadvantage associated with high temperatures above1,050° C. that reaction between the investment compounds and the glassmaterials can occur and even the investment compounds themselves startdeterioration in strength especially for those glass materials ofphosphate and cristobalite systems.

In general, the glass material has a softening point (Td) of at least500° C. and a glass transition temperature (Tg) of 400 to 900° C.

Desirably the glass material can undergo a distortion or deformation of20% or more, even 30% or more, and in some cases 60% or more at atemperature of preferably up to 1,000° C., more preferably up to 900° C.and a pressure of preferably up to 20 MPa, more preferably up to 5 MPa,further preferably up to 1 MPa, most preferably up to 0.1 MPa.

The above-mentioned distortion behavior and the relationship ofsoftening point and crystallization temperature of glass material can bereadily accomplished by suitably selecting a composition from theabove-mentioned range.

The shape and size of glass material may be suitably determineddepending on a particular application. For example, artificial dentalcrowns have an easy to mold shape and to this end, a frustoconical,cylindrical or spherical shape may be employed. To prevent entry ofbubbles during molding, to prevent lowering of mechanical strength, andto produce a homogeneous molded body, a single piece of glass materialis preferably used in the manufacture of a single molded body.Differently stated, it is recommended to avoid a plurality of glassmaterial pieces or glass powders being integrally joined during molding.However, two or more glass material pieces may be used as part of theinvention if necessary.

It is not intended in the present invention that a flux or plasticizerbe added to glass material for softening purposes upon pressure molding.

Preparation

According to the present invention, a living tissue replacement isprepared by pressure molding, or pressure molding and machining, ormachining of glass material.

The method utilizing pressure molding is described in detail, with itsvariants illustrated in FIG. 1.

The glass material is prepared by melting a raw material and quenchingthe melt. For melting, the raw material is heated in a crucible ofplatinum, quartz or alumina for about 10 seconds to 20 hours, preferablyabout 1 minute to about 2 hours. The melting temperature which dependson the composition is generally above 1,400° C. The raw material is amixture of oxides or substances capable of forming oxides upon melting,for example, carbonates, bicarbonates and hydroxides. Upon heating, rawmaterial components react with each other to form a composite oxide.Melting is generally done in air. The quenching method is not criticalinsofar as amorphous glass is obtained at the end of quenching. Forexample, the melt is poured to sheet iron, carbon, water or mold. Themold may be made of dental investment materials such as cristobalite,calcium phosphate or the like.

For glass homogenization, a melting-cooling-crushing-remelting processmay be repeated or high-frequency induction heating may be employed.

According to the invention, the thus prepared amorphous glass materialis converted into a living tissue replacement of crystallized glass,preferably by any of the procedures shown in FIGS. 1(a) to 1(e).

The procedure shown in FIG. 1(a) includes the steps of nucleating theglass material, pressure molding it at a temperature equal to or aboveits glass transition temperature, and crystallizing the glass materialat the same time as or after the pressure molding step.

The procedure shown in FIG. 1(b) includes the steps of pressure moldingthe glass material at a temperature equal to or above its glasstransition temperature and crystallizing the glass material at the sametime as or after the pressure molding step.

The procedure shown in FIG. 1(c) includes the steps of pressure moldingthe glass material at a temperature equal to or above its glasstransition temperature, thereafter nucleating the glass material, andcrystallizing it.

The procedure shown in FIG. 1(d) includes the steps of nucleating theglass material, crystallizing it, and thereafter pressure molding theglass material at a temperature equal to or above its glass transitiontemperature.

The procedure shown in FIG. 1(e) includes the steps of crystallizing theglass material and thereafter pressure molding it at a temperature equalto or above its glass transition temperature.

In these procedures, nucleation treatment is optionally effected inorder to allow for uniform crystallization of glass. Although a certaincomposition of glass material fails to provide a desired color tone andsufficient strength due to abnormal crystal growth, nucleation treatmentprior to crystallization ensures uniform crystallization. Thennucleation treatment is advantageously used when the invention isapplied to dental crowns for which outer appearance is of importance.The nucleation treatment is also effective for reducing the time takenin crystallization. The conditions of nucleation treatment are notcritical although heat treatment is preferably effected at a temperaturenear the nucleation temperature, especially the nucleation temperature±50° C. for about 10 minutes to about 30 hours. Usually the holdingtemperature is about 250 to about 900° C., preferably about 400 to about800° C. If the nucleation temperature is close to the crystallizationtemperature, nucleation treatment may be omitted without substantialinfluence.

Essential in the procedures of FIG. 1 is pressure molding which uses atemperature equal to or above the glass transition temperature of glassmaterial, preferably a temperature at which molding is possible under apressure of up to 20 MPa. With a proper composition chosen from theabove-mentioned range, pressure molding is possible under a pressure ofup to 20 MPa even if the glass material has a crystallinity as high as50% by volume. However, if the crystallinity is up to 30% by volume,more preferably up to 8% by volume, most preferably up to 4% by volume,the glass material undergoes good flow during pressure molding, allowinga molded part of complex shape to be readily produced to precision. Withtoo high crystallinity, the glass material has an extremely increasedviscosity and is almost impossible to mold. In order to utilize theviscous flow of glass material for pressure molding, the glass materialis preferably heated to a temperature at which a viscosity of lower than10⁹ poise is reached. Such a lower viscosity facilitates molding. Sincecrystallization of glass material can proceed during molding dependingon the temperature of pressure molding, the temperature during moldingis suitably selected such that the crystallinity may fall within thedesired range. This temperature varies with the time taken for moldingand may be determined empirically, and most often the temperature islower than the crystallization temperature +50° C. (T≦Tx+50° C.). Toprevent excessive crystallization, it is preferred not to maintain thematerial near the crystallization temperature for a long time, forexample, more than about 1 hour. It is to be noted that a definitecrystallization temperature is not available under certain conditions,for example, when the heating rate is high or low. That is, melting canstart without crystallization. In such a case, glass material is subjectto pressure molding at a temperature in the range where the glassmaterial does not melt. Preferably in order to avoid partial melting ofglass material, the temperature of glass material does not exceed 0.8times the melting point (T≦0.8×mp). The heating temperature at whichpressure molding is carried out is at or above the glass transitiontemperature (Tg) and preferably at or above the softening point becausebelow the softening point, the glass material shows insufficient flow tomold to a complex shape.

As will be described later, the living tissue replacement of theinvention requires only a crystallinity of at least 10% by volume. Then,when the glass material is crystallized to a crystallinity of 10 to 50%by volume prior to pressure molding as in FIGS. 1(d) and 1(e),crystallization treatment following the pressure molding can be omitted.It is acceptable in these procedures that further crystallization takesplace during pressure molding. Also, in the procedures of FIGS. 1(a) and1(b), it is possible to simultaneously perform crystallization to 10 to50% by volume and pressure molding.

The glass material can be molded, for example, by placing it in a moldand pressing it by a punch. The mold and punch may be made of dentalinvestment materials based on cristobalite and phosphate cristobalite,alumina and zirconia. The mold and punch may be prepared with theordinary skill of dentists and dental technicians.

The glass material may be heated by placing it in a pre-heated mold orby placing it in a mold and introducing the mold into a furnace. Afterthe glass material has been heated to the predetermined temperature, itis subject to pressure molding. A hot press technique may be used forheating and pressing purposes. In an alternative technique, once heated,the mold is taken out of the furnace and a pressure force is thenapplied. This technique is effective for improving productivity becausea plurality of molds each loaded with glass material can be concurrentlyheated in the furnace. Application of pressure force to the glassmaterial may be initiated either before or after the maximum temperatureduring molding is reached. The former is advantageous in shortening themolding step because molding begins at the same time as the glassmaterial softens. If a necessary deformation is achieved before themaximum temperature is reached, pressurization may be interrupted atthat point of time, which contributes to a further time saving. Thelatter ensures homogeneity after molding because the glass maintains aconstant viscosity during pressurization. Since a pressure force isapplied subsequent to a glass viscosity drop, the mold is prevented fromfracture. A pressure force is maintained until the glass material hasbeen deformed to faithfully reflect the mold cavity, often for about 5to 20 minutes although the exact time varies with press means andtemperature.

At the end of molding, the pressure force is preferably released whilethe glass material remains above its glass transition temperature, morepreferably while the glass material temperature is above its softeningpoint. If the pressure force is maintained after the temperature haslowered below the glass transition temperature, that is, the glassmaterial has hardened, the pressure force is transmitted to the moldwhich can be cracked or even broken and sometimes, the glass materialitself can be cracked.

No particular limitation is imparted to the pressing technique duringmolding. Since the glass material can be molded under a low pressure ofup to 20 MPa, especially up to 1 MPa, the invention eliminates a needfor special press means and enables molding by merely using a hand pressor weight, for example. In the case of a weight, the mold is heated withthe weight rested on the punch. Then the weight descends as the glassmaterial lowers its viscosity. The completion of molding can be detectedby the termination of downward displacement of the weight. Also, when apressure force is applied by a press machine with a constant crossheadspeed maintained, the completion of molding may be determined in termsof crosshead displacement or pressure increase.

FIGS. 2 to 5 illustrate exemplary pressure molding procedures. Anapparatus for shaping a living tissue replacement by pressure moldingincludes a mold 2 and a punch 4. The mold 2 includes a molding cavity 21and a bore 23 which receives the punch 4 and is connected for fluidcommunication to the cavity 21 through a sprue 22. The cavity 21 is of adental crown shape in the illustrated examples. A frame 5 is an outerframe which is used when the mold 2 is cast. A buffer 6 lined inside thecasting frame 5 is to accommodate expansion of the mold material. Oftenthe casting frame 5 is an iron ring, and the buffer 6 is an asbestosribbon. The molding cavity 21, sprue 22 and bore 23 are defined by aconventional lost wax process or the like. A block of glass material 3is placed at the bottom of the bore 23 and compressed by the punch 4 inthe arrow direction. Since the glass material has been heated to apredetermined temperature and thus has a low viscosity, the appliedpressure causes the glass material to flow into the cavity 21 throughthe sprue 22 where it deforms faithfully to the cavity 21 to assume thedental crown shape.

In the embodiments shown in FIGS. 2 and 3, the bore 23 has a taperedinner surface. The bore inner surface is inwardly tapered, preferably ata gradient of up to 1/5, more preferably up to 1/15. In the preferredembodiment shown in FIG. 4, the bore 23 has a straight inner surface.The bore inner surface extends substantially parallel to the pressureapplication direction shown by the arrow. The bore with a slightlytapered or straight inner surface defines with a similarly tapered orstraight punch 4 a minimal gap which is effective for preventing backflow of the softened glass material 3 therethrough. This saves theinitial amount of glass material to be loaded. The bore 23 is generallycircular in cross section (taken perpendicular to the pressureapplication direction) although it may also be ellipsoidal or polygonal.

Preferably the mold 2 is provided with a vent or run-off path 24connected in fluid communication between the molding cavity 21 and theexterior as shown in FIGS. 3 to 5. The vent 24 allows part of the glassmaterial to run off upon pressure application, thereby facilitating tofill the cavity 21 with the glass material and to produce a molded partfaithful to the cavity under a relatively low pressure. The vent 24 isalso effective for preventing application of excess pressure to the moldupon pressure application, thus preventing the mold 2 from cracking. Asa result, the percentage of deficient parts is drastically reduced. Thediameter and cross-sectional shape of the vent 24 may be suitablydetermined depending on the volume and shape of the molding cavity 21.More than one vent 24 may be provided. Often the vent 24 is defined by aconventional lost wax process or the like. In the illustratedembodiments, the vent 24 opens to the exterior of the mold 2. Sinceconventional dental molds are well air permeable, it is unnecessary todischarge air in the molding cavity 21 through the vent 24. Then thevent 24 need not open to the exterior of the mold 2. The vent 24 iscommunicated to the mold exterior in the illustrated embodiments becausesuch a through-vent is easier to define by a lost wax process.

Although the sprue 22 connecting the molding cavity 21 and thepunch-receiving bore 23 is aligned with the pressure applicationdirection shown by the arrow in the embodiments shown in FIGS. 2 to 4,the sprue 22 is preferably inclined with respect to the pressureapplication direction as shown in FIG. 5. The inclined sprue 22facilitates to channel the glass material into the cavity 21 and toproduce a molded part faithful to the cavity under a relatively lowpressure and prevents the mold 2 from cracking upon pressureapplication. The angle of inclination relative to the pressureapplication direction is not particularly limited although it is desiredthat when the inlet and outlet of the sprue 22 are projected to a planeperpendicular to the pressure application direction, their projectedimages do not overlap. As long as the inlet and outlet of the sprue 22are offset in this relationship, the sprue 22 need not be a straight oneas shown in FIG. 5, with a curved sprue acceptable.

The sprue 22 preferably has a cross-sectional shape corresponding to theshape of the molding cavity 21. When it is desired to mold an artificialdental crown for the incisor as in the illustrated embodiment, themolding cavity 21 has a rectangular shape with high flatness in a crosssection perpendicular to the sprue 22 and hence, the sprue 22 shouldalso have a corresponding rectangular shape with a flatness of about 1:7in cross section. Then a molded body of a shape highly faithful to themolding cavity 21 can be produced under a relatively low pressure. Thesprue 22 need not have a constant cross-sectional area from the inlet tothe outlet and may vary in area.

In a further preferred embodiment, the mold 2 includes a liner 25 whichcovers at least a portion of the inner surface of the punch-receivingbore 23 as shown in FIGS. 10 and 11. The liner 25 is made of areinforcing or high strength material having a higher compressionstrength than the material of the mold 2. The bore 23 is lined with thehigh strength liner 25 at its inner surface for preventing rupture ofthe molded part. Since pressure is applied to the mold 2 during pressuremolding, the mold 2 can be cracked. If the glass material penetratesinto such cracks, there is a likelihood that the molding cavity 21 beshort of the glass material. Still worse, if cracks are communicated tothe cavity 21, the resulting molded part has burrs. Such cracking can beavoided by increasing the strength of the mold with a concomitantfailure of the molded part upon withdrawal from the mold. This isbecause the molded part is generally withdrawn from the mold by applyingan external force to the mold to form cracks therein and removing themold material fragments. To destroy the high strength mold, a greaterforce must be applied to the mold so that the inside molded part can bedamaged thereby. Then in the embodiments shown in FIGS. 10 and 11, thebore 23 is provided with the high strength liner 25 on the inner surfaceand the mold 2 itself is made of a material having relatively lowcompression strength, which not only prevents the mold from crackingunder the molding pressure, but also ensures that the mold is readilydestroyed without causing damage to the inside molded part uponwithdrawal of the molded part from the mold.

Although the high strength liner 25 is preferably extended over theentire inner surface of the bore 23 as shown in FIG. 10, it is onlyrequired that the liner 25 cover a portion of the bore inner surface,especially that portion of the bore inner surface which comes in contactwith the glass material 3 during pressure molding. Since it is ratherdifficult and expensive to apply high strength material to the entirebore inner surface to form the liner 25 shown in FIG. 10, it isacceptable that the high strength material or liner be not applied nearthe bottom of the bore 23 as shown in FIGS. 11 and 12.

The high strength liner 25 is generally about 0.1 to about 3 mm thickalthough the thickness is not critical and may be suitably determined bytaking into account the type of high strength material and moldingpressure.

The punch 4 receives only compression stress in a substantial sense.Then even when the punch 4 is made of a material having relatively lowcompression strength, its failure is less probable. However, it ispreferred that the punch 4 is at least partially formed of a reinforcingor high strength material 45 having higher compression strength than thematerial of the mold 2 as shown in FIGS. 13(a), 13(b) and 13(c). Thehigh strength material or cover 45 prevents the punch 4 from suchfailure as cracks and fracture. The reinforcement or cover 45 of highstrength material forms or covers at least a portion of the surface ofthe punch 4 opposed to the bore inner surface, especially that portionof the punch surface which comes in contact with the glass material 3during pressure molding. Since there is a tendency that some glassdeposits remain on the punch 4 after molding, it is a common practice toreplace the punch 4 by a new one on every molding operation. Then theembodiment wherein only a portion of the punch 4 is made of the highstrength material as shown in FIG. 13 is cost effective because thepunch can be manufactured at a lower cost by reducing the amount of thehigh strength material which is expensive than the low strengthinvestment materials. The high strength cover 45 in the embodiments ofFIGS. 13(b) and 13(c) may have a thickness similar to that of the liner25 associated with the bore 23.

The material of which the mold 2 is made preferably has a compressionstrength of up to 20 MPa, more preferably up to 15 MPa and preferably atleast 2 MPa, more preferably at least 4 MPa at the end of pressuremolding. If the compression strength of the mold material is too high,the probability of molded part failure would become high for theabove-mentioned reason. If the compression strength of the mold materialis too low, the mold would be broken during molding even when the highstrength liner is provided.

The compression strength of the mold material is defined herein as thatafter compression molding because the mold is concurrently heated duringpressure molding and the compression strength depends on this heating.It is possible to facilitate withdrawal of the molded part from the moldby immersing the mold in water for softening and in this case, thecompression strength is that of the mold which has been immersed inwater.

Also preferably the high strength material for the liner 25 (FIGS.10-12) and the cover 45 (FIG. 13) has a compression strength of at least15 MPa, more preferably at least 30 MPa. With a compression strengthbelow the limit, provision of the reinforcing material would bemeaningless. No upper limit need be imposed on the compression strengthof the reinforcing material. Often a material having a compressionstrength of up to 2,000 MPa is preferably used for availability and easeof shaping.

The compression strength used herein is measured according to JIS R 1608when the mold material and reinforcing material are ceramics. Moreparticularly, five cylindrical samples of 12.5 mm in diameter and 5 mmheight are molded from the material and measured for compressionstrength at a crosshead speed of 0.5 mm/min. For a metallic reinforcingmaterial, the compression strength is given as the value at which anobject on test is broken when the same compression strength measuringprocedure as used for the ceramics is carried out.

The material having relatively low compression strength of which themold 2 is made may be suitably selected from dental investmentmaterials, for example, cristobalite and phosphate system cristobalitessuch as calcium phosphate cristobalite as well as gypsum, with thecristobalite being preferred. Cristobalite may be softened, furthersoftened by immersing in water, or smoothed on surface. The highstrength material for the liner 25 or reinforcement 45 is notparticularly limited and may be suitably selected by taking into accountits relationship to the compression strength of the mold material,preferably from metal and ceramic materials. Ceramic materials areespecially preferred because heat during pressure molding can causemetals to react with the glass material to undesirably color the glassmaterial. Preferred examples of the metal are stainless steel and ironwhile preferred examples of the ceramic include alumina, siliconcarbide, zirconia, and zeolite. Ceramic mixtures such as mixtures ofvarious porcelains and refractories (e.g., feldspar-quartz-kaolinsystems) are also preferred. Further phosphate system cristobalites,dental refractories and gypsum are acceptable.

Preferred investment compounds are of cristobalite and gypsum systems,such as OK Powder commercially available from Shofu K.K. As opposed toother castable ceramics, investment compounds of cristobalite and gypsumsystems, which ensure facile removal of molded parts and improvedsurface properties can be used because the molding temperature isrelatively low.

Any desired method may be used to form the liner 25 and reinforcement orcover 45 of high strength material although the following method isoften used.

FIG. 14 illustrates how to form the mold 2 by a lost wax process. Thecasting frame 5 and buffer 6 are rested on a support 7. A shape 8 ofsilicone rubber or the like for forming the punch-receiving bore isplaced within the frame 5. A high strength material is applied to thesurface of the shape 8 to form the liner 25. On the shape 8 is located awax shape 9 for forming the sprue and cavity. In this state, a moldmaterial such as an investment compound is cast into the frame 5followed by ordinary steps of a conventional lost wax process.

The punch 4 including a lower portion in the form of the high strengthmaterial reinforcement 45 shown in FIG. 13(a) is prepared, as shown inFIG. 15, by placing a high strength material 45 in a cavity of apunch-forming mold 10 of silicone rubber or the like, casting a punchmaterial such as an investment compound thereon, and removing the mold10.

After the glass material is pressure molded by the living tissuereplacement molding apparatus mentioned above, the molded part isallowed to cool down inside or outside the furnace. If desired, themolded part is slowly cooled at a controlled rate. The molded part canbe taken out of the mold by an ordinary technique commonly employed bythe dentist or dental technician.

Crystallization treatment is made on the molded part of glass materialby holding the molded part approximately at the crystallizationtemperature, preferably in the range between the crystallizationtemperature minus 200° C., more preferably the crystallizationtemperature minus 100° C. and the crystallization temperature plus 50°C. (Tx-200° C.≦T≦Tx+50° C.). The holding time is not critical and issuitably determined such that a desired crystallinity may be obtained.Often the holding time is within 10 hours, preferably within 3 hours. Itis also acceptable to omit temperature holding, that is, to startcooling immediately after the predetermined temperature is reached.

Where crystallization treatment follows the pressure molding step, it isrecommended to subject the molded part to crystallization treatmentwithout cooling because the overall process becomes efficient.

Crystallization can be done without taking the molded part out of theinvestment compound after molding. In contrast, conventional castableceramics have a likelihood that the glass be broken because the moldedpart must be removed prior to crystallization and if crystallization isdone without removal, the molded part be cracked due to differentialexpansion from the investment compound. The present invention enablescrystallization within the mold without cracking because of minimalthermal expansion.

The molded part after crystallization has a structure whereincrystalline phase is dispersed in the vitreous matrix. The proportion ofcrystalline phase, that is, crystallinity is not particularly limitedalthough it is preferably at least 10% by volume, more preferably 20 to100% by volume. A glass material with too low crystallinity would beinadequate as the dental crown because of insufficient mechanicalstrength and high clarity. Also undesirably, when a projectioncorresponding to the sprue is removed, the fracture surface would becomesharp. It is to be noted such a projection corresponding to the sprue isremoved by abrasion.

The resulting crystals are shown in the ternary phase diagram of FIG. 7.In a preferred range of composition according to the present invention,there is mainly created diopside: (Ca,Mg)O--MgO--2SiO₂, preferablyCaO--MgO--2SiO₂. In a more preferred range of composition, there issubstantially solely created diopside. Diopside preferably occupies atleast 30% by volume, more preferably at least 70% by volume, mostpreferably at least 80% by volume of the entire crystalline matter.Besides diopside there are created other crystals includingwollastonite: β--(Ca,Mg)O--SiO₂, especially CaO--SiO₂, alite: 3CaO--SiO,belite: 2CaO--SiO₂, akermanite: 2CaO--MgO--2SiO₂, monticellite:CaO--MgO--SiO₂, forsterite: 2(Mg,Ca)O--SiO₂, protoenstatite:(Mg,Ca)O--SiO₂, and tridymite: SiO₂. Preferred among these areakermanite and/or monticellite.

The crystallinity used herein can be determined by a peak separationtechnique using an X-ray diffraction chart. In an X-ray diffractionchart of crystallized glass having crystals dispersed in a vitreousmatrix, there appear a halo indicative of the presence of vitreousmatter and inherent peaks corresponding to a particular crystallinematter. The peak separation technique determines an integral intensityas a sum of only peak areas and an overall integral intensity as a sumof peak and halo areas and divides the former by the latter to calculatea crystallinity.

At the end of crystallization, the average grain size generally rangesfrom about 0.001 to about 100 μm, preferably up to 1 μm, more preferablyup to 0.5 μm. It is difficult to provide a grain size smaller than thisrange whereas high strength would not be expected from a too large grainsize. The grain size is determined by measuring the area of each grainin a photomicrograph under a scanning electron microscope (SEM) andcalculating the diameter of a circle corresponding to the area.

In the present invention, the glass transition temperature, softeningpoint, crystallization temperature and nucleation temperature may bedetermined by differential thermal analysis and measurement of acoefficient of thermal expansion. It is to be noted that since the glasstransition temperature, softening point, and crystallization temperaturedo not substantially alter after nucleation, the heating temperatureused for the pressure molding of a previously nucleated glass materialmay be determined on the basis of the glass transition temperature andsoftening point of a non-nucleated glass material. The same applies tothose glass materials which have been crystallized such that pressuremolding may be done under a pressure of less than 20 MPa as shown inFIGS. 1(d) and 1(e).

Living tissue replacements such as artificial dental crowns may bedirectly prepared by pressure molding of glass material as mentionedabove. It is also possible that the molded part resulting from pressuremolding be machined prior to completion of a living tissue replacement.The machining step is advantageous in producing a living tissuereplacement of complex shape to which a mold cavity can be preciselymolded with difficulty or in producing a living tissue replacement whichrequires very high dimensional precision. If the molded part has a shapeand size approximate to the final living tissue replacement, machiningcan be completed within a short time and the waste of glass material canbe reduced. Alternatively, it is possible to directly machine a block ofglass material without pressure molding. There may be used any ofmachining techniques including machining using drills of high hardnessmaterial such as diamond and carborundum, lathes and the like.

Where the living tissue replacement is an artificial dental crown, it isgenerally stained after crystallization.

Although the invention has been described as being applied to artificialdental crowns, the invention is equally applicable to other livingtissue replacements, for example, artificial bones such as ossiculum,bone screws, percuteneous terminals, blood vessels, and air tubes. Suchexamples of the living tissue replacement are illustrated in FIG. 6.FIG. 6(a) is an artificial vertebra body, FIG. 6(b) is an artificialintervertebral body, FIG. 6(c) is an artificial iliac bone, FIG. 6(d) isan artificial air tube, and FIG. 6(e) is a bone screw.

EXAMPLE

Examples of the present invention are given below by way of illustrationand not by way of limitation.

Examples 1-10 & Comparative Examples 1-3

Glass Material

Guaranteed reagents CaCO₃, SiO₂, and MgO (available from Kanto ChemicalK.K.) were weighed as shown in Table 1 and milled in a vibratory millwith zirconia balls for one hour. Each mixture was placed in a 30-ccplatinum crucible and heated in an electric furnace model SuperBurn(manufactured by Motoyama K.K.) for one hour to form a molten glass. Themelting temperature is shown in Table 1.

The molten glass was cast into a dental investment material CosmotecVest (manufactured by GC K.K.) for cooling and annealed at 750° C.,obtaining a glass material. Some of the thus prepared glass materialsamples had a metal or metal compound added thereto as shown in Table 1.The content of the additive is expressed in percent by weight based onthe total of CaO+SiO₂ +MgO=100% by weight.

These glass material samples as prepared were measured for meltingtemperature, nucleation temperature and crystallization temperature (Tx)by differential thermal analysis and for glass transition temperature(Tg) and softening point (Td) by a thermal expansion/contraction test.The differential thermal analysis was carried out by crushing the glassmaterial in an alumina mortar, weighing a 65-mg portion therefrom, andheating the sample at a rate of 10° C./min. by a differential thermalanalysis meter (manufactured by Mac Science Co.). The thermalexpansion/contraction test used a specimen of 3×4×35 mm having surfacesmirror finished with diamond paste and a thermal expansion/contractionmeter (manufactured by Shimazu Mfg. K.K.) operating at a heating rate of5° C./min. The results are shown in Table 1. It is to be noted that theglass material samples were also measured for melting point bydifferential thermal analysis to find that all the samples had a meltingpoint of higher than 1,400° C.

Nucleation Treatment

Some of the glass material samples were subject to nucleation treatmentby heating at the nucleation temperature shown in Table 1 for 8 hours inan electric furnace model SuperBurn (manufactured by Motoyama K.K.).Whether or not the samples were subject to nucleation treatment is shownin Table 1.

Pressure Molding

A mold and punch for pressure molding were prepared from a dentalinvestment material Cosmotec Vest (manufactured by GC K.K.) having acompression strength of about 50 MPa. A frustoconical block (height 5mm, maximum diameter 5 mm, minimum diameter 4 mm) of the glass materialwas placed in the bore of the mold, which was heated at a rate of 20°C./min. in a split electric furnace SS-1700 (manufactured by Nems K.K.).After a soaking time of 10 minutes passed, the glass material waspressure molded in the furnace. By means of a strength tester Instron1350 (manufactured by Instron) with a constant crosshead speed of 0.5mm/min., a pressure was applied to the glass material in its axialdirection to achieve molding within a maximum displacement of 3 mm.Provided that the distortion of glass material corresponds to thedisplacement of the crosshead, a load-distortion curve was recorded. Theapplied pressure was calculated according to the equation:

    S=p/A

wherein p is the load, A is the minimum cross-sectional area of theglass material before deformation, and S is the applied pressure. Themolding pressure reported in Table 1 implies the applied pressure toeffect a distortion of 25%, the distortion being a deformation of glassmaterial divided by the height of glass material before deformation.

                                      TABLE 1                                     __________________________________________________________________________    Glass-forming raw material composition                                        __________________________________________________________________________    Base composition                                                              (total 100 wt %)                                                                 CaO   SiO.sub.2                                                                        MgO  Additive (wt % based on base composition)                    __________________________________________________________________________    E 1                                                                              26    55.8                                                                             18.2                                                              E 2                                                                              26    55.8                                                                             18.2                                                              E 3                                                                              26    55.8                                                                             18.2                                                              CE 1                                                                             26    55.8                                                                             18.2                                                              CE 2                                                                             26    55.8                                                                             18.2                                                              E 4                                                                              26    55.8                                                                             18.2 Ag (0.01)                                                    E 5                                                                              26    55.8                                                                             18.2 Au (0.01) + Fe.sub.2 O.sub.3 (0.2) + ZrO.sub.2 (3)           E 6                                                                              26    55.8                                                                             18.2 Ag (0.01) + Fe.sub.2 O.sub.3 (0.2) + ZrO.sub.2 (3) +                          CeO.sub.2 (0.05) + TiO.sub.2 (10)                            E 7                                                                              32    51.5                                                                             16.5 Ag (0.01) + CeO.sub.2 (0.5)                                  E 8                                                                              22.6  69.4                                                                             8    Au (0.01) + TiO.sub.2 (1.0) + CeO.sub.2 (0.5)                E 9                                                                              40    45 15   Au (0.01) + CeO.sub.2 (0.5)                                  E 10                                                                             20.3  55.2                                                                             24.5 Au (0.01) + Fe.sub.2 O.sub.3 (0.2)                           CE 3                                                                             45.5  50 4.5  Au (0.005) + CeO.sub.2 (0.1)                                 __________________________________________________________________________       Melting                  Molding                                                                            Crystallinity                                                                        Molding                                  temp.                                                                             Nucleation                                                                          Nucleation                                                                         Tg Td Tx  temp.                                                                              after molding                                                                        pressure                                 (° C.)                                                                     Temp. (° C.)                                                                 treatment                                                                          (° C.)                                                                    (° C.)                                                                    (° C.)                                                                     (° C.)                                                                      (%)    (MPa)                                 __________________________________________________________________________    E 1                                                                              1450                                                                              550   none 732                                                                              782                                                                              870 750  0      15                                    E 2                                                                              1450                                                                              550   none 732                                                                              782                                                                              870 800  0      0.3                                   E 3                                                                              1450                                                                              550   none 732                                                                              782                                                                              870 850  0      <0.01                                 CE 1                                                                             1450                                                                              550   none 732                                                                              782                                                                              870 900  60     rupture                               CE 2                                                                             1450                                                                              550   none 732                                                                              782                                                                              870 950  80     rupture                               E 4                                                                              1450                                                                              550   treated                                                                            732                                                                              782                                                                              870 850  0      <0.01                                 E 5                                                                              1450                                                                              500   treated                                                                            732                                                                              782                                                                              870 850  0      <0.01                                 E 6                                                                              1450                                                                              500   treated                                                                            740                                                                              782                                                                              870 850  0      <0.01                                 E 7                                                                              1450                                                                              500   treated                                                                            730                                                                              805                                                                              860 830  0      <0.01                                 E 8                                                                              1450                                                                              500   treated                                                                            725                                                                              780                                                                              840 800  0      <0.01                                 E 9                                                                              1450                                                                              500   treated                                                                            766                                                                              810                                                                              880 900  6      <0.01                                 E 10                                                                             1550                                                                              500   treated                                                                            730                                                                              770                                                                              840 800  0      <0.01                                 CE 3                                                                             1500                                                                              500   treated                                                                            790                                                                              850                                                                              1050                                                                              900  50     rupture                               __________________________________________________________________________

The glass materials of Examples 1-3 and Comparative Examples 1-2 had acoefficient of thermal expansion of 5.97×10⁻⁶ /°C. in the temperaturerange between 400° C. and 700° C.

The relationship of applied pressure and displacement during pressuremolding in Example 3 is illustrated in FIG. 8. It is seen thatdeformation took place under a pressure of 0.008 MPa (about 80 g/cm²) to0.04 MPa (about 400 g/cm²) until a displacement of 2 mm was reached.

The relationship of applied pressure and displacement during pressuremolding in Examples 2-3 and Comparative Examples 1-2 is illustrated inFIG. 9. It is seen that at 800° C. and 850° C. which are higher than thesoftening point (782° C.) and lower than the crystallization temperature(870° C.), deformation took place under a pressure of lower than 3 MPa(about 30 g/cm²) and lower than 0.1 MPa (about 1 g/cm²), respectively,even at a displacement of about 3 mm (corresponding to a distortion ofabout 60%). At temperatures of 900° C. and 950° C., the specimens wereruptured on the way of increasing the applied pressure. The rupturedfragments were whitened and had a crystallinity of more than 60% byvolume. This implies that pressure molding at 900° C. or higher inducedexcess crystallization which inhibited sufficient deformation.

Pressure Molding Outside the Furnace

The glass material was prepared by the same procedure as Example 3,subject to nucleation treatment, and then molded by the followingprocedure.

In an electric furnace KDFVR7 (manufactured by Ring Furnace) set at 850°C., a pressure molding mold and punch assembly loaded with the glassmaterial was placed and held at the temperature for 10 minutes.Immediately after the assembly was taken out of the furnace, that is,before the temperature of the glass material began lowering, pressurewas applied to the glass material by means of a press machine Shot Cure2, Jiyo No. 52 (manufactured by Towa Giken K.K.). The pressure necessaryfor molding was the same as in Example 3.

Casting

Glass material was prepared as in Example 5 and molded by the followingprocedure.

In an electric furnace model SuperBurn (manufactured by Motoyama K.K.)operating at 1,450° C. was placed a mold and punch assembly loaded withthe glass material. The glass material was melted by heating for 10minutes. Immediately after the assembly was taken out of the furnace,that is, before the temperature of the glass material began lowering,pressure was applied to the molten glass material by means of a pressmachine Shot Cure 2, Shiyo No. 52 (manufactured by Towa Giken K.K.).

In this procedure, the glass reacted with the mold and the molten glassflowed back and did not fill the mold cavity to its extremity. Bubblesand cracks were observed in the molded part. Due to substantialshrinkage upon cooling, the part largely deviated from the desireddimensions.

After molding or casting, the part was subject to nucleation treatmentand crystallization treatment, resulting in insufficient nucleation andnon-uniform crystallization. Although the molded part prepared inExample 5 was tinted to a desired yellow color at the end ofcrystallization, the cast part did not possess the desired color, but adeeper color despite the same composition.

Examples 11-20 & Comparative Example 4

Each of glass materials as shown in Table 2 was heat treated in anelectric furnace KDFVR7 (manufactured by Ring Furnace) forcrystallization, obtaining a test specimen. The heating rate was 5°C./min. while the holding temperature and time are shown in Table 2.Examples 19-20 used the glass material which had the same composition asExample 5, but had not been nucleated. Precipitated phases of diopside(D) and akermanite (A) were identified by powder X-ray diffractometryanalyzer XD-D1 (manufactured by Shimazu Mfg. K.K.).

The specimens were measured for flexural strength. The specimens were ofthe same dimensions as used for the thermal expansion/contraction testand had a similarly mirror finished surface. The flexural strength wasmeasured by a three-point bend test using a strength tester ServoPulserEHF-F1 (manufactured by Shimazu Mfg. K.K.) at a crosshead speed of 0.5mm/min. and a span of 15 mm. The number of test specimens was 5 for eachlot. For comparison purposes, the glass material which had the samecomposition as Example 1, but was not crystallized (Comparative Example4) was also measured for flexural strength. The results are shown inTable 2.

                  TABLE 2                                                         ______________________________________                                               Crystallization                                                                         Flexural                                                     Glass    Temp.   Time    strength                                                                            Precipitated                                                                          Outer                                  material (° C.)                                                                         (hr.)   (MPa) phase   appearance                             ______________________________________                                        CE 4 E 1*    --      --    120   --      clear                                E 11 E 1*    870     6.0   300   diopside                                                                              white                                E 12 E 4     870     0.5   300   diopside                                                                              white                                E 13 E 5     870     1.0   300   diopside                                                                              yellow**                             E 14 E 6     870     1.0   300   diopside                                                                              yellow**                             E 15 E 7     860     0.1   280   diopside                                                                              yellow**                             E 16 E 8     840     0.5   250   diopside                                                                              yellow**                             E 17 E 9     880     0.5   200   diopside/                                                                             yellow**                                                              akermanite                                   E 18 E 10    840     0.5   250   diopside                                                                              yellow**                             E 19 E 5*    890     5.0   300   diopside                                                                              yellow**                             E 20 E 5*    870     10.0  300   diopside                                                                              yellow**                             ______________________________________                                         *The glass materials used in CE 4, E 11, E 19, and E 20 had not been          nucleated.                                                                    **natural tooth color                                                    

As seen from Table 2, Examples 11, 19 and 20 which had not beennucleated required a substantially extended time for crystallization.The crystallized materials of these examples had a crystallinity of 30to 50% by volume. In Example 17, diopside occupied 50% by volume of theprecipitated phase.

Example 21

The procedure of Example 13 was repeated except that two blocks of glassmaterial were placed in the bore upon pressure molding. Some voids wereleft in the molded part.

Comparative Example 5

The procedure of Example 13 was repeated except that the glass materialused was a 200-mesh under powder. Many voids were left in the moldedpart.

Comparative Example 6

A glass material was prepared by melting a mixture of 24.8% CaO, 16.9%MgO, 16.3% SiO₂, 22.8% TiO₂, 15.7% P₂ O₅, 2.0% CaF₂, 1.0% Al₂ O₃, and0.5% ZrO₂, all in percent by weight, at 1,350° C. and casting the melt.The glass material was pressure molded at 750° C. and 20 MPa, but nosufficient deformation occurred. The glass material was heat treated at840° C. for crystallization to find precipitation of apatite and MgTiO₃.The crystallized material had a flexural strength of 120 MPa.

Comparative Example 7

A glass material was prepared by melting a mixture of 35% SiO₂, 15% B₂O₃, 15% Al₂ O₃, 20% MgO, 2.5% K₂ O, 7.5% Na₂ O, and 5% F, all in percentby weight, at 1,350° C. and casting the melt. The glass materialfractured when pressure molded at 750° C.

Examples 22-30

Glass materials containing additives as shown in Table 3 were preparedas in Examples 1-10. The base composition was the same as in Example 1.These glass materials had physical properties as shown in Table 3.

The glass materials were pressure molded. Some glass materials weresubject to nucleation treatment by the same procedure as described aboveprior to pressure molding. Whether or not the nucleation treatment waseffected is reported in Table 3 together with the nucleationtemperature. A mold and punch for pressure molding were made of a dentalinvestment material Cosmotec Vest (manufactured by GC K.K.). Afrustoconical block (height 5 mm, maximum diameter 5 mm, minimumdiameter 4 mm) of the glass material was placed in the bore of the mold,which was heated at a rate of 20° C./min. in a split electric furnaceSS-1700 (manufactured by Nemus K.K.). After a soaking time of 10 minutespassed, the glass material was pressure molded in the furnace. Themolding pressure determined by the method mentioned above was less than5 MPa for all the glass materials.

Each of glass materials was heat treated in an electric furnace KDFVR7(manufactured by Ring Furnace) for crystallization, obtaining a testspecimen. The heating rate was 5° C./min. while the holding temperatureand time are shown in Table 3.

Each sample was examined for precipitated phase, outer appearance,flexural strength, fracture toughness (K_(IC)) and semi-transparency.The results are shown in Table 3. The measurement of fracture toughnessused a sample of the same dimensions as used in the flexural strengthtest and having at the center a notch of 0.75 mm deep and 100 μm wide.The semi-transparency was evaluated by placing a sample on printedmatter and rated "⊚" when the printed characters were readable with easeand "∘" when readable with some difficulty. Good semi-transparencyensures an outer appearance closely resembling natural teeth afterstaining.

                                      TABLE 3                                     __________________________________________________________________________    Additive to glass material                                                    (wt % based on Melting                                                                           Nucleating    Molding                                      base composition)                                                                            temp.                                                                             temp.                                                                              Tg Td Tx temp.                                           TiO.sub.2                                                                        Ag.sub.2 O                                                                       ZrO.sub.2                                                                        CeO.sub.2                                                                        (° C.)                                                                     (° C.)                                                                      (° C.)                                                                    (° C.)                                                                    (° C.)                                                                    (° C.)                                __________________________________________________________________________    E 22                                                                              1 0.5                                                                              -- -- 1500                                                                              700  740                                                                              770                                                                              880                                                                              810                                          E 23                                                                              5 0.5                                                                              -- -- 1500                                                                              none 753                                                                              780                                                                              890                                                                              810                                          E 24                                                                             10 0.5                                                                              -- -- 1500                                                                              none 770                                                                              800                                                                              890                                                                              810                                          E 25                                                                             13 0.5                                                                              -- -- 1500                                                                              none 770                                                                              800                                                                              890                                                                              810                                          E 26                                                                             15 0.5                                                                              -- -- 1500                                                                              none 785                                                                              805                                                                              900                                                                              810                                          E 27                                                                             20 0.5                                                                              -- -- 1500                                                                              750  785                                                                              805                                                                              900                                                                              810                                          E 28                                                                             -- -- 15 0.5                                                                              1500                                                                              800  730                                                                              790                                                                              890                                                                              850                                          E 29                                                                             -- --  5 0.5                                                                              1500                                                                              600  720                                                                              780                                                                              880                                                                              850                                          E 30                                                                             -- 0.5                                                                              10 -- 1500                                                                              770  730                                                                              790                                                                              870                                                                              850                                          __________________________________________________________________________    Crystallization                                                                          Flexural                                                              Temp.                                                                             Time                                                                              strength                                                                          Precipitated                                                                        Outer K.sub.IC                                                                           Semi-                                            (° C.)                                                                     (min.)                                                                            (MPa)                                                                             phase appearance                                                                          (MPam.sup.1/2)                                                                     transparency                                  __________________________________________________________________________    E 22                                                                             820 30  275 diopside                                                                            yellow*                                                                             2.5  ⊚                              E 23                                                                             820 30  260 diopside                                                                            yellow*                                                                             2.5  ⊚                              E 24                                                                             820 20  260 diopside                                                                            yellow*                                                                             2.5  ⊚                              E 25                                                                             825 20  250 diopside                                                                            yellow*                                                                             2.4  ⊚                              E 26                                                                             825 20  240 diopside                                                                            yellow*                                                                             2.3  ∘                                 E 27                                                                             825 20  240 diopside                                                                            yellow*                                                                             2.3  ∘                                 E 28                                                                             820 30  210 diopside                                                                            yellow*                                                                             2.0  ∘                                 E 29                                                                             820 40  200 diopside                                                                            yellow*                                                                             2.0  ∘                                 E 30                                                                             830 30  220 diopside                                                                            yellow*                                                                             2.2  ∘                                 __________________________________________________________________________     *natural tooth color                                                     

In the examples shown in Table 3, the parts had a crystallinity of 0% byvolume at the end of pressure molding and a crystallinity of 30 to 50%by volume at the end of crystallization. In Table 3, Examples 24 to 27containing 10 to 20% by weight of TiO₂ and 0.5% by weight of Ag₂ Orequired only a very short time for crystallization.

A glass material sample was prepared from the same glass material asExample 25 shown in Table 3 except that the crystallization conditionswere changed as shown in Table 4. These samples were measured forphysical properties and examined for semi-transparency and color tone.For comparison purposes, a non-crystallized sample of the samecomposition was similarly examined. The results are shown in Table 4.

                  TABLE 4                                                         ______________________________________                                        Physical properties of crystallized glass (Composition of Example 25)         Crystallizing conditions                                                                   omitted 825° C./10 min.                                                                    850° C./45 min.                       ______________________________________                                        Precipitated phase                                                                         --      diopside    diopside                                     Density (g/cm.sup.3)                                                                       3.03    3.10        3.13                                         Shrinkage factor (%)                                                                       --      0.66        1.0                                          Coefficient of thermal                                                                     5.9     5.9         6.4                                          expansion (10.sup.-6 /° C.)                                            Flexural strength (MPa)                                                                    120     200         250                                          Fracture toughness                                                                         0.7     1.7         2.4                                          (PMam.sup.1/2)                                                                Young's modulus (GPa)                                                                      32.9    34.0        34.9                                         Vickers hardness                                                                           640     680         720                                          Crystallinity (vol %)                                                                      0       20          40                                           Semi-transparency                                                                          --      ⊚                                                                          ∘                                Color tone   --      pale yellow pale yellow                                  Mean grain size (μm)                                                                    --      0.05        0.1                                          ______________________________________                                    

Examples 31-36

Glass materials of the composition shown in Table 5 were prepared as inExamples 1-10. The amounts of additives are expressed in percents byweight relative to the total of CaO+SiO₂ +MgO=100% by weight. The glasstransition temperature (Tg), crystallization temperature (Tx) andmelting point (mp) of the glass materials are also shown in Table 5. Theglass materials were subject to heat treatment (1) and heat treatment(2) as shown in Table 5 in this order for crystallization. Thecrystallized glass materials had a crystallinity of 30 to 70% by volumeand the crystalline phase was of diopside. The glass material sampleswere of the same dimensions as in Examples 11 to 20. The crystallizedglass materials were measured for Vickers hardness, flexural strength,machinability, color tone, luster and semi-transparency. The measuringmethods and evaluation criteria are shown below.

Vickers Hardness

A Vickers hardness meter was used.

Flexural Strength

Measurement was done as in Examples 11-20.

Machinability

A sample was perforated with a carborundum drill with a diameter of 1.5mm. The sample was rated "⊚" when drilling was quite easy, "∘" wheneasy, and "Δ" when drilling was possible.

Color Tone

A sample was rated "⊚" when it had a color identical with natural teeth,and "∘" when close to natural teeth.

Luster

A sample was rated "⊚" when it had a luster identical with naturalteeth, and "∘" when close to natural teeth.

Semi-transparency

Evaluation was the same as in Examples 22-30.

The results are shown in Table 5.

Comparative Example 8

A hydroxyapatite (HAP) sample of the same dimensions as in Examples 31to 36 was prepared and similar measurement and evaluation were done. Theresults are also shown in Table 5.

Comparative Example 9

A titanium (Ti) sample of the same dimensions as in Examples 31 to 36was prepared and similar measurement and evaluation were done. Theresults are also shown in Table 5.

                                      TABLE 5                                     __________________________________________________________________________    Base composition                                                                              Additives (wt % based                                         (total 100 wt %)                                                                              on base composition)                                                                         Tg  Tx   mp                                        CaO MgO SiO.sub.2                                                                         TiO.sub.2                                                                         ZrO.sub.2                                                                         others (° C.)                                                                     (° C.)                                                                      (° C.)                         __________________________________________________________________________    E 31                                                                              25.9                                                                              18.5                                                                              55.6                                                                              13.0                                                                              1.4 Ag.sub.2 O (0.7)                                                                     770 920  1400                                  E 32                                                                              25.9                                                                              18.7                                                                              55.4                                                                              13.9                                                                              0   Ag.sub.2 O (0.7)                                                                     770 890  1400                                  E 33                                                                              25.9                                                                              18.5                                                                              55.6                                                                               7.4                                                                              5.8 AgCl (0.7)                                                                           770 950  1400                                  E 34                                                                              25.9                                                                              18.5                                                                              55.6                                                                              13.9                                                                              0.6 Ag.sub.2 O (0.7)                                                                     770 910  1400                                  E 35                                                                              25.0                                                                              21.5                                                                              53.5                                                                              12.7                                                                              0.9 AuCl.sub.3 (0.5)                                                                     770 930  1400                                  E 36                                                                              25.9                                                                              18.5                                                                              55.6                                                                               9.4                                                                              1.0 Ag.sub.2 O (0.5)                                                                     770 940  1400                                  CE 8                                                                              HAP Sintered body                                                         CE 9                                                                              Ti                                                                        __________________________________________________________________________    Heat treatment                                                                           Heat treatment                                                     (1)        (2)          Flexural       Light                                     Temp.                                                                             Time                                                                              Temp.                                                                             Time                                                                              Vickers                                                                            strength                                                                          Machin-    trans-                                    (° C.)                                                                     (min.)                                                                            (° C.)                                                                     (min.)                                                                            hardness                                                                           (MPa)                                                                             ability                                                                           Color                                                                            Luster                                                                            mittance                               __________________________________________________________________________    E 31                                                                             770 20  850 30  550  370 ⊚                                                                  ⊚                                                                 ⊚                                                                  ⊚                       E 32                                                                             770 20  810 60  600  320 ∘                                                                     ⊚                                                                 ⊚                                                                  ⊚                       E 33                                                                             770 20  890 30  640  220 ∘                                                                     ∘                                                                    ∘                                                                     ∘                          E 34                                                                             770 20  850 30  570  350 ⊚                                                                  ⊚                                                                 ⊚                                                                  ⊚                       E 35                                                                             770 20  810 60  500  350 ⊚                                                                  ⊚                                                                 ⊚                                                                  ⊚                       E 36                                                                             770 20  830 60  540  340 ⊚                                                                  ⊚                                                                 ⊚                                                                  ⊚                       CE 8                                                                             --  --  --  --  370  100 Δ                                                                           x  x   ∘                          CE 9                                                                             --  --  --  --  --   --  Δ                                                                           x  x   x                                      __________________________________________________________________________

The crystallized glass material samples of Examples 24, 25, 26, 28, 29,and 30 were also examined for machinability by the same procedure asabove. They were all rated excellent.

Molding Assembly with Reinforcement

Using a mold having a high strength liner in the cavity as shown in FIG.11 and a punch having a reinforcement as shown in FIG. 13(a) or a punchmade solely of high strength material, pressure molding was done as inExample 1. The materials for the mold and punch and the high strengthmaterials for the mold liner and punch reinforcement are shown in Table6. For comparison purposes, pressure molding was similarly done using amold and punch of the same material (Combination Nos. 8 and 9). Thecompression strength (CS) of these materials are shown in Table 6.

                                      TABLE 6                                     __________________________________________________________________________    Combi-                                                                        nation                                                                             Mold    CS   Mold liner                                                                             CS   Punch      Punch reinforce-                   No.  Material                                                                              (MPa)                                                                              Material (MPa)                                                                              material   ment material                      __________________________________________________________________________    1    Univest (Silky)                                                                       ˜12                                                                          iron     *    Uni vest   iron                               2    Snow white                                                                             ˜8                                                                          stainless steel                                                                        *    Snow white stainless steel                    3    Ceravest G                                                                            ˜10                                                                          alumina  ˜2000                                                                        Ceravest G alumina                            4    Ceravest G                                                                            ˜10                                                                          Cosmotec Vest II                                                                        ˜50                                                                         Cosmotec Vest II                                                                         --                                 5    Ceravest G                                                                            ˜10                                                                          P.L.V. (smile)                                                                          ˜35                                                                         P.L.V.     --                                 6    Ceravest G                                                                            ˜10                                                                          zeolite  ˜100                                                                         Ceravest G zeolite                            7    Cristobalite Q                                                                         ˜6                                                                          porcelain                                                                              ˜150                                                                         Cosmotec Vest II, CS                                                                     ˜50 MPa                      8**  Cosmotec Vest II                                                                      ˜50                                                                          --            Cosmotec Vest II                                                                         --                                 9**  Uni vest (Silky)                                                                      ˜12                                                                          --            Uni vest   --                                 __________________________________________________________________________     *not ruptured under a pressure of 20 MPa                                      **comparison                                                             

Using a mold and punch assembly of the combination shown in Table 6,pressure molding was repeated 100 times. Combination No. 8 wherein boththe mold and punch were made solely of high compression strengthmaterial (Cosmotec Vest II) was free of cracks, but when the molds weredestroyed with a tool for withdrawal of the molded parts, some partswere ruptured. Occurrence of ruptured parts was 5%. In combination No. 9wherein both the mold and punch were made solely of low compressionstrength material (Univest), cracks occurred in the mold where glasscould penetrate, resulting in burrs on the molded part. Occurrence ofdefective parts was 5%.

In combination Nos. 1 to 7 wherein the mold and punch satisfied thegeneral and preferred requirements of the invention, neither rupture ofthe molded parts nor cracking of the molds was observed.

Using the mold and punch mentioned above, the following compositionswere press molded by the same procedure as Example 1.

                  TABLE 7                                                         ______________________________________                                        Vitrification             Press molding                                       SiO.sub.2                                                                          CaO    MgO    Temp.          Glass temp.                                                                           Molding                             ______________________________________                                        63.0 29.0   8.0    1500° C.                                                                      almost  1000° C.                                                                       OK                                                            crystallized                                        61.0 27.0   12.0   1500° C.                                                                      almost   950° C.                                                                       OK                                                            vitrified                                           52.0 41.0   7.0    1500° C.                                                                      almost   980° C.                                                                       NO*                                                           crystallized                                        ______________________________________                                         *The glass flowed a little, but did not proceed through the sprue.       

The data show that the composition of JP-B 36107/1992 is not prone topress molding.

Similarly, a composition composed of 55.6% of SiO₂, 25.9% of CaO, and18.5% of MgO and having added thereto 10% of TiO₂ and 0.5% of AgNO₃,based on 100% by weight of the SiO₂ /CaO/MgO base, was press molded atdifferent temperatures using the following investment compound.

                  TABLE 8                                                         ______________________________________                                        Molding conditions                                                                        Observation    Investment compound                                ______________________________________                                         910° C./20 min.                                                                   satisfactorily molded/                                                                       Cosmotec Vest                                                  crystallized                                                       980° C./10 min.                                                                   satisfactorily molded/                                                                       Cosmotec Vest                                                  crystallized                                                      1050° C./10 min.                                                                   molded, but reacted                                                                          Cosmotec Vest                                                  with investment                                                   ______________________________________                                    

The glass reacted with the investment compound at a temperature of 1050°C., resulting in a rough surface, bubbles and cracks.

The benefits of the invention are evident from the foregoing data.

Japanese Patent Application Nos. 5-139079, 5-214944, 5-353680, and6-80966 is incorporated herein by reference.

Although some preferred embodiments have been described, manymodifications and variations may be made thereto in the light of theabove teachings. It is therefore to be understood that within the scopeof the appended claims, the invention may be practiced otherwise than asspecifically described.

We claim:
 1. A method for preparing a molded glass materialcomprising:(1) obtaining a glass material prepared by quenching a moltenmass of glass raw materials, (2) pressure molding the glass material ata temperature equal to or above its glass transition temperature butbelow its melting point, and at a pressure of up to 20 MPa, and (3)effecting crystallization treatment on the glass material during orafter the pressure molding to produce a crystallinity of at least 10% byvolume in said molded glass material, wherein said glass material isselected from those from which at least one of CaO--MgO--2SiO₂(diopside), CaO--MgO--SiO₂ (monticellite), 2CaO--MgO--2SiO₂(akermanite), 2(Mg,Ca)O--SiO₂ (forsterite), β-(Ca,Mg)O--SiO₂(wollastonite), Na₂ O--Al₂ O₃ --2SiO₂ (nephelite), Na₂ O--Al₂ O₃ --6SiO₂(albite), Na₂ O--Al₂ O₃ --4SiO₂ (jadeite), MgO--TiO₂, Al₂ O₃ --SiO₂(andalusite), 3Al₂ O₃ --2SiO₂ (mullite), CaO--Al₂ O₃ --2SiO₂(anorthite), 2CaO--Al₂ O₃ --SiO₂ (gehelenite), and 3CaO--Al₂ O₃ --3SiO₂(grossularite) can precipitate out, and wherein apatite cannotprecipitate out.
 2. The method of claim 1, wherein said glass materialprior to pressure molding has a first crystallinity and said moldedglass material has a second crystallinity greater than said firstcrystallinity.
 3. The method of claim 1 wherein said glass material hasa crystallization temperature and a softening point which is lower thanthe crystallization temperature and exhibits viscous flow attemperatures below its melting point.
 4. The method of claim 1 whichfurther comprises the step of nucleating the glass material prior to thecrystallization treatment.
 5. The method of claim 1 which furthercomprises nucleating the glass material prior to pressure molding. 6.The method of claim 1 wherein the pressure molding is carried out at atemperature which is not higher than 50° C. above the crystallizationtemperature of said glass material.
 7. The method of claim 1 wherein theglass material during said pressure molding has a crystallinity of up to50% by volume.
 8. The method of claim 1 wherein said pressure molding iscarried out at a temperature which is up to 0.8 times the melting pointof the glass material.
 9. The method of claim 1 wherein the glassmaterial during said pressure molding has a viscosity of up to 10⁹poise.
 10. The method of claim 1 wherein at the end of said pressuremolding of the glass material, the pressure is released while the glassmaterial is at a temperature not lower than its glass transitiontemperature.
 11. The method of claim 1 wherein the pressure molding iscarried out in a molding apparatus including a mold and a punch, saidmold including a mold body and a molding cavity and a bore through whichthe punch is inserted, the punch having a surface opposed to the boreinner surface, the bore being in fluid communication with said moldingcavity through a sprue, whereinthe glass material is placed in the boreand the punch is urged into the bore against the glass material foreffecting pressure molding in the molding cavity.
 12. The method ofclaim 11 wherein the step of effecting crystallization treatment on saidglass material is carried out in the molding cavity.
 13. The method ofclaim 11 wherein said mold further includes a vent in fluidcommunication with said molding cavity.
 14. The method of claim 11wherein said bore is defined by said inner surface which extendssubstantially parallel to the direction in which pressure during thepressure molding is applied.
 15. The method of claim 14 wherein saidbore inner surface has a gradient of up to 1/5 relative to the pressureapplying direction.
 16. The method of claim 14 wherein said mold furtherincludes a liner covering at least a portion of the bore inner surface,said liner being made of a high strength material having a highercompression strength than the mold body.
 17. The method of claim 16wherein a cover forms at least a portion of the surface of said punchopposed to the bore inner surface, said cover being made of a highstrength material having a higher compression strength than the moldbody.
 18. The method of claim 17 wherein said mold body has acompression strength of up to 20 MPa at the end of said pressuremolding.
 19. The method of claim 17 wherein both said high strengthmaterials have a compression strength of at least 15 MPa.
 20. The methodof claim 16 wherein said mold body has a compression strength of up to20 MPa at the end of said pressure molding.
 21. The method of claim 16wherein said high strength material has a compression strength of atleast 15 MPa.
 22. The method of claim 11 wherein said sprue is inclinedrelative to the pressure applying direction.
 23. The method of claim 11wherein said sprue has a cross-sectional shape corresponding to thecross-sectional shape of said molding cavity.
 24. The method of claim 1which further includes the step of machining the glass material afterthe crystallization treatment.
 25. The method of claim 1, wherein theglass material is a non-calcium phosphate system composition comprisingsilicon oxide, calcium oxide, and magnesium oxide.
 26. The method ofclaim 1, wherein the glass material comprises 40-70 weight percent ofSiO₂, 20-50 weight percent of CaO, and 8-30 weight percent of MgO. 27.The method of claim 1, wherein said pressure molding is carried out at apressure of up to 5 MPa.
 28. The method of claim 1, wherein saidpressure molding is carried out at a pressure of up to 1 MPa.
 29. Themethod of claim 1, wherein said pressure molding is carried out at apressure of up to 0.1 MPa.